Durham All-Sky Camera (DAC)

Abstract

In this work, the prototype of a Durham all-sky camera (DAC) was designed and constructed for the purpose of monitoring the night sky of Durham. The DAC consisted of a Fujinon fish-eye lens coupled to a monochrome 640-by-480-pixel CCD camera (DMK-21BF04 model from the ImagingSource) housed in an 8-inch Perspex dome. After construction, the images were taken of the night sky in order to determine the properties of DAC. The field of view of the DAC was 185˚ at average scale of 0.2˚ per pixel.

The astrometric characteristics of the camera were investigated by measuring the relationship between star positions on the 3-D celestial hemisphere and their projected 2-D pixel positions on the DAC CCD images. The derived relationship (the mapping) of the stars onto the CCD images achieved an average uncertainty of 1 pixel. For the reverse process, the uncertainties were 0.2˚ in elevation and 0.7˚ in azimuth. The relationship was tested for the robustness and was found to be stable at the level of 1 pixel.

The photometric characteristics of the camera were studied by investigating how well the magnitude of a star could be measured by DAC. The results showed that, under the sky background condition in Durham, the camera was able to determine a 6th magnitude star at zenith within 1 magnitude uncertainty, but a 4th magnitude star within the similar uncertainty when the stars were at an elevation of 30˚. The brightness of the sky background of Durham was determined to be 18 magnitudes per square arcsecond. Subsequently, the comparison was drawn between the sky background in Durham and in Hawaii, thereby estimating that the accuracy of measuring a 6th magnitude star would be about 7 times better if the camera was deployed under the darker sky background of Hawaii.